A lithium-ion battery (also called Li-ion battery or LIB) is a rechargeable battery in which lithium ions travel from the negative electrode to the positive electrode during discharge, and in the opposite direction during charge. Lithium-ion batteries have attracted considerable attention due to their high energy density, high efficiency, no-memory effect, comparatively low weight, long life and environmentally friendly operation. They are today widely used for example for portable electronics, as well as in various types of electrical or hybrid vehicles.
Lithium-ion batteries are in general composed of a positive electrode, a negative electrode, an electrolyte and optionally a separator. For the electrodes, electrode active material(s) and conductive additive, such as carbon, are “glued” together by a binder and provided as a coating on a current collector. The electrode active materials for the positive electrode are lithium storage compounds, such as lithium cobalt oxide (LiCoO2), lithium iron phosphate (LFP) or a lithium containing spinel such as lithium manganese oxide (LMO). Olivine LiFePO4 has attracted more and more interest due to its advantages in terms of reasonably high theoretical capacity (170 mAh/g), flat voltage plateau 3.5 V vs. Li, relatively low cost, high safety and long cycling life. The electrode active material of the negative electrode may for example be graphite and/or a lithium storage compound.
The conductive additive material is usually carbon or carbon-based, and often in the form of carbon black, graphite or carbon fibers.
The conventional binder material for electrodes in commercial Li-ion batteries is polyvinylidene fluoride (PVDF), which has strong binding strength with current collectors as well as the electrode active material. However, all fluorinated polymers can easily deteriorate cycling performance of the batteries because of the formation of stable LiF and double bond (C═CF—) after reacting with Li metal and lithiated graphite (LixC6). In addition, the arising heat generation using PVDF as binder could cause self-heating thermal runaway, which increases the safety issue. Furthermore, PVDF needs to be dissolved in organic solvent, typically N-methyl-2-pyrrolidinone (NMP), to produce a slurry to be used for the coating process of the current collector. During the subsequent drying procedure, NMP, which is expensive, toxic and harmful to both humans and environment, evaporates into the air.
Therefore, efforts have been made to develop alternative binders that are non-fluorinated, less costly and more environmental friendly. Recently, much effort has focused on using aqueous binders instead of non-aqueous binders for electrodes for Li-ion batteries, which could meet the requirements above. Water-based binders, such as styrene-butadiene-rubber (SBR), sodium-carboxyl-methyl-cellulose (CMC), poly(acrylamide-co-diallyldimethylammonium) (AMAC), and elastomer, have been evaluated as low cost green binder materials for electrodes of Li-ion batteries and show similar bonding ability and high flexibility. However, CMC, an extremely stiff and brittle polymer, suffers from inferior mechanical properties for optimized electrodes. Furthermore, SBR often suffers from inferior flexibility and cycle life, and may also have less than ideal adhesion.
US 2012/0225199 A1 discloses a coating for a current collector for a rechargeable electro-chemical cell comprising an electrode active material, a conductive additive material and a water-soluble polymeric binder. A large number of various water-soluble polymeric binders are proposed, including for example poly vinyl alcohols, polyvinyl pyrrolidone, polyethylene oxides, as well as starch, cellulose, protein, polysaccharide, dextrans, tannin and lignin.
It has also previously been proposed to use lignin-based composites to prepare carbon based materials for lithium-ion batteries or to use lignin-based carbon fibers as free-standing anode material.
CN 102361071 A1 discloses a method for preparing a lithium ion battery anode material. Lignin salt made of lignin ammonium sulfonate, sodium lignosulfonate, calcium lignosulfonate or lignin sulfonic acid magnesium is used as carbon source precursor of the anode material.